# Set parity of a string

Peter Hansen peter at engcorp.com
Sun Jan 23 21:00:25 EST 2005

```Peter Hansen wrote:
> snacktime wrote:
>> Is there a module that sets the parity of a string?
>
> As to the specific question: a module is not really required.

But here's one for you anyway.  It raises an exception if any
input character is non-ASCII, otherwise when you call set_parity()
with a string and a parity parameter of 'even', 'mark', 'none',
'odd', or 'space', it returns a string of the same length with
the high (parity) bits set appropriately.

'''parity module for python'''
# Released under the "MIT license"

import string
import operator

def _makeTable(parity):
bitMap = {'even': [0,128], 'odd': [128,0], 'mark': [128,128]}
table = []
for c in range(128):
even_odd = (sum(bool(c & 1<<b) for b in range(8))) & 1
cp = chr(c | bitMap.get(parity, [0,0])[even_odd])
table.append(cp)
table.extend(chr(c) for c in range(128, 256))
table = ''.join(table)
return table, table[128:]

_map = {}
for parity in 'even odd mark space none'.split():
_map[parity] = _makeTable(parity)

def set_parity(data, parity='none'):
'''return string with parity bits, accepting only ASCII characters
(0..127) as input'''
out = string.translate(data, *_map[parity.lower()])
if len(out) != len(data):
raise ValueError('non-ASCII characters in input')
else:
return out

if __name__ == '__main__':
print 'Running self-tests for parity.py'

# check for invalid input handling
for input in [ '\xff', '\x80', 'test\xA0ing' ]:
try:
set_parity(input)
except ValueError:
pass
else:
assert False, 'no exception for non-ASCII input'

# check for various valid inputs
for i, (parity, input, expected) in enumerate([
('space', 'test', 'test'),
('SPACE', '\x00\x7f', '\x00\x7f'),
('mark', 'test', '\xf4\xe5\xf3\xf4'),
('Odd', '\x00test\x7f', '\x80\xf4\xe5s\xf4\x7f'),
('even', '\x00\x01\x02\x03test\x7d\x7e\x7f',
'\x00\x81\x82\x03te\xf3t\x7d\x7e\xff'),
]):
actual = set_parity(input, parity)
assert expected == actual, 'case %d: %r != %r' % (i, expected, actual)

print 'All tests passed.'

```